Direct RF is a newly emerging field in wireless digital communications wherein analog RF signals that are transmitted over-the-air are directly sampled into a digital data stream suitable for digital signal processing. A typical wireless digital communications device would use analog filters, duplexors, mixers, analog-to-digital converters (ADC), etc. to convert the analog RF signals into a digital data stream that is suitable for digital signal processing. Unfortunately, analog circuit components, especially components such as capacitors, inductors, resistors, etc. necessary for the analog filters are difficult to integrate into an integrated circuit. This is especially true for the precise values of these components required for use in filters. Of course, the desire of the manufacturer is to maximize the degree of integration for the wireless transceivers. This is because the more highly integrated a wireless transceiver can become, the lower the production costs for the transceiver and the transceiver will typically use less power during operation.
Direct RF involves the direct conversion of the analog RF signal into digital data stream through the use of a direct sampling mixer, without having to undergo any intermediate analog filtering, downconversion, etc. An example of a direct RF sampling mixer is one that uses current to perform its sampling. The current-mode direct sampling mixer converts the received analog RF signal into a current that is then integrated by a sampling capacitor. The charge on the sampling capacitor is then periodically read out to produce the discrete-time data stream. The discrete-time data stream is then converted to a digital data stream by a quantizer or an analog-to-digital converter (DAC).
A common source of impairment or potential impairment that may occur whenever a signal is sampled at a particular sampling rate (or frequency) is aliasing. When a signal is sampled at a particular sampling frequency, Fs, then any part of the signal at a frequency that is greater than Fs/2 will wrap around and combine with the signal at frequencies less than Fs/2. This wrapping around of frequency components greater than Fs/2 is known as aliasing. Therefore, to accurately represent a signal with a certain bandwidth, F, the sampling must be performed at a frequency of at least 2*F. This is known as the Nyquist rate for the signal.
When the signal being sampled has frequency components at frequencies greater than one half of the Nyquist rate, these frequency components are aliased down into the signal and as a result, interference and/or noise is added to the sample stream. A typical way to reduce the aliasing is to use filters, commonly referred to as anti-aliasing filters, to remove any frequency components beyond one half of the Nyquist rate.
However, anti-aliasing filters may be rather complex filters that are difficult to design and integrate onto an integrated circuit. They can be difficult to design if the desired frequency response is sharp. Furthermore, they can be both bulky (consuming a large amount of real estate) and expensive (due to requirements for precise component values). In integrated circuits, it is often not possible to include an anti-aliasing filter on the same integrated circuit containing the sampling circuit and digital circuitry, therefore these filters may be external to the integrated circuit. This requires that the signals go off-chip, reducing the overall performance of the system and introducing additional complexity. Additionally, anti-aliasing filters are static in nature, meaning that their frequency response is set when they are designed and fabricated and cannot be changed to meet changing spectral environments; For example, should operating conditions change such that a formerly low-powered, frequency component that was aliasing into a band of interest but had been previously ignored due to its low power, now for some reason becomes a significant source of interference, a static anti-aliasing filter will not be able to adapt to eliminate that particular aliasing frequency component. Note that the term frequency band of interest may represent a single communications channel or a group of communications channels, typically adjacent.
A need has therefore arisen for a method and apparatus that is capable of detecting aliasing frequency components, determine their effect to the signal within the frequency band of interest, and eliminate the aliasing frequency components.